The cited DC loop may also include the telephone and may control as mentioned earlier the DC voltage between the two subscriber line wires too.
The DC loop may therefore often provide the following functions:
Sets the subscriber line current feed characteristics PA1 Generates the ring signal PA1 Senses the ring signal PA1 Detects the ringtrip PA1 Detects the loop
The DC loop also determines the internal battery and the DC feed resistance to the line.
The DC feed characteristic is determined by sensing the line voltage and then the DC loop controls the line current through the loop. In FIG. 1 a simplified diagram of the DC loop is shown during a ring burst mode.
The line current I.sub.L has the following equation: ##EQU1##
From the expression above, the output impedance of the circuit towards the line becomes: ##EQU2##
where
I.sub.L =line current
U.sub.L =line voltage (voltage difference between A- and B-wire)
Z.sub.F =line feed impedance,
.omega.=pole corner frequency, 1,5 Hz DC-loop filter, alternatively
170 Hz in ring burst mode,
GR=line current is GR times the current through RDC,
RDC=external resistor,
R.sub.1 =internal resistor sets line feed impedance,
I.sub.bat =current generator and
gm=transconductance factor
The internal battery voltage is determined by the current generator Ibat and the transconductance factor gm. ##EQU3##
The current generator Ibat is constant in all the modes except the ring burst mode.
During the ring burst mode, when the ring signal is on, the ring signal is superimposed on the internal battery voltage. This voltage is generated by a constant current. The ring signal is achieved by superimposing an AC-current on the constant current. The current generator Ibat shapes thus the ring signal and the battery voltage may form a square ring signal as in FIG. 2.